In direct-injection spark-ignition engines (Otto engines) and common rail diesel engines, fuel is typically injected directly into the combustion chamber of the cylinders by electronically controlled injection valves. The fuel is typically supplied by an electric feed pump from the fuel tank and stored at high-pressure in an accumulator volume upstream of the injection valves (injectors). The pressure in the accumulator volume is generated by a high-pressure fuel pump which is mechanically driven by the internal combustion engine, in particular by the camshaft or crankshaft. In spark-ignition engines, the typical fuel pressure in idle mode is approximately 60 bar and during normal operation approximately 120 bar. Conversely, diesel engines have operating pressures of at least approximately 300 bars in idle mode, and of maximally approximately 1800 bar in normal driving mode. With this type of injection, an injection pressure can be generated independent of the motor RPM and the injected fuel quantity.
The typical startup operation of internal combustion engines is performed by conventional, battery-operated starter motors, which are activated by turning an ignition key and the like and transfer a torque to the crankshaft of the internal combustion engine. At the same time, fuel injection is activated. As soon as the internal combustion engine reaches a ceiling RPM, from which the engine can run-up on its own (for spark-ignition engines approximately 60 . . . 100 min−1, for diesel engines approximately 80 . . . 200 min−1), the starter motor is disengaged from the crankshaft and the internal combustion engine continues to run up to its engine-dependent idle RPM. This method has the disadvantage that the injection pressure in direct-injection internal combustion engines is relatively low during the first injection events, because the high-pressure fuel pump is driven mechanically, generally only slightly above the pressure of the feed pump of approximately 4 to 7 bar. This adversely affects the quality of the fuel jet and the combustion and hence also the exhaust gas emission (in particular when the catalytic converter system has not yet reached its operating temperature). For example, at injection pressures below approximately 10 bar, a narrow jet is formed on the injectors instead of the desired finely dispersed spray cone. Incomplete processing of the mixture in the combustion chamber during startup operation is conventionally at least partially compensated by increasing the injected fuel quantity, which results in increased emissions.
Another problem during the startup phase of the direct-injection spark-ignition engine or diesel engine is partial condensation of the injected fuel on the cylinder walls and the piston head while these are still cold. The wall film evaporates and only partially combusts, while the rest is exhausted in the form of HC emissions. In order to compensate for the undesirable “starving” (becoming lean) due to the formation of the wall film and to produce a combustible mixture, a larger fuel quantity is usually injected than required after the engine has warmed up. This fuel enrichment in the startup phase or after the startup phase also causes increased emissions.
Also known are hybrid drives for motor vehicles which include an (for example, direct injection) internal combustion engine and in addition at least one electric machine which can be selectively switched to a motor mode or a generator mode. With serial hybrid concepts, the vehicle is driven exclusively by the electric machine, while the internal combustion engine generates via a separate generator electric current for charging an energy storage device that supplies the electric machine or for directly supplying current to the electric machine. Conversely, parallel hybrid concepts are at present frequently used at least in passenger vehicle applications, wherein the vehicle can be driven by the internal combustion engine or the electric machine, or both. In parallel concepts, the electric machine is typically connected in motor mode to support the internal combustion engine while operating at higher vehicle loads. The electric machine can also operate as a starter motor (“starter generator”) for the internal combustion engine. However, the electric machine is predominantly operated as a generator while the vehicle is powered by the internal combustion engine, whereby the generated electric energy is used to charge the energy storage device and/or to supply energy to an onboard electric system. In addition, at least a portion of the braking power is supplied by the electric machine in generator mode (energy recovery) by converting a portion of the dissipated mechanical energy into electric energy. The internal combustion engine of a hybrid drive is typically equipped with a start-stop automatic, which controllably turns the internal combustion engine off and restarts the internal combustion engine under certain conditions. Frequent startup of the direct-injection internal combustion engine in hybrid drives only aggravates the aforedescribed problems.
In summary, the startup phase of direct-injection internal combustion engines consumes a relatively large quantity of fuel and increases emissions, which due to its rate of recurrence has adverse effects particularly in hybrid drives.